To permanently delete a modern group, you'll need to access SharePoint online from PowerShell:

1. Download and install the latest version of the SharePoint Online Management Shell from https://go.microsoft.com/fwlink/p/?LinkId=255251.
2. Launch PowerShell and import the SharePoint Online module:

Import-Module SharePointOnline

3. Once it has been imported, you'll need to connect to SharePoint Online:

Connect-SPOService -Credential (Get-Credential) -Url https://<tenant>-admin.sharepoint.com

4. Retrieve the list of deleted sites, and select the site to permanently delete:

Get-SPODeletedSite

5. From the list of deleted sites, select the value under the URL that represents the site you wish to permanently delete:

6. Use that value to specify which site to remove:

Remove-SPODeletedSite -Identity https://<tenant>.sharepoint.com/sites/<sitename>